The invention relates generally to an instrument for moisture determination.
More particularly, the invention relates to an electrical instrument for measuring the moisture content of granular solids, e.g. grain, as well as non-granular solids such as, for example, wood, hardened or set structural materials, and insulating materials.
A known electrical instrument for determining the moisture content of a material functions to measure the electrical resistance of the material. The electrical resistance is converted into a moisture reading. The instrument has a sensor which is contacted with the material in order to measure the electrical resistance of the latter. The instrument is further provided with a suitable indicator for indicating the moisture content of the material. An amplifier, and preferably a logarithmic amplifier, is disposed between the sensor and the indicator to amplify the signals transmitted by the sensor. Furthermore, a calibrating circuit is located between the amplifier and the indicator. The calibrating circuit, which functions to adjust the instrument to the particular material being measured, includes a potentiometer which is connected with the output of the amplifier. The output or tap of the potentiometer is connected with the indicator. Advantageously, a second amplifier is disposed between the potentiometer and the indicator. The potentiometer is adjustable from externally of the instrument.
As indicated above, the amplifier which is connected with the input of the potentiometer is preferably a logarithmic amplifier. The reason is that the characteristic curve representing the absolute moisture content of the material as a function of the electrical resistance is logarithmic in nature. By using a logarithmic amplifier, the signals generated by the sensor are thus converted to a linear function over a wide range of values, i.e. the characteristic curve becomes linear over a wide range of values.
The characteristic line representing the absolute moisture content as a function of the measured resistance is different for different types of materials and also for different materials of the same type. For example, the characteristic lines for grain and wood are different. In addition, the characteristic lines for different forms of grain are different as are the characteristic lines for different forms of wood such as pine, beech, and so on.
The calibrating circuit outlined above makes it possible to measure the moisture content of different materials, e.g. different forms of wood, with a single instrument. To this end, the calibrating circuit enables the moisture measuring instrument to operate along a series of characteristic lines. In this manner, the instrument may at least approximate the individual characteristic lines of the different forms of a particular type of material.
The potentiometer of the calibrating circuit operates to effect a proportional change in the readings of the measuring instrument and thereby causes the slope of the characteristic line of the instrument to change. This permits a first approximation to the characteristic line of a given material to be achieved. However, a first approximation is inadequate in many instances. Thus, the range of adjustment obtainable through a change in slope is not sufficiently wide to take into account all different forms of a particular type of material, and the characteristic lines of certain forms may lie relatively far outside of the adjustment range so that the approximation by the measuring instrument is relatively poor. The situation is even worse when the instrument is to approximate the characteristic lines of different types of materials, e.g. grain and wood, because the deviation of the characteristic lines of different types of materials from an average range or field of characteristic lines is greater than that for different forms of the same type of material. In addition, the characteristic lines for different types of materials may have a different appearance.